Linear Technology DC2247A Demo Manual

Description
Demonstration circuit DC2247A is a 36V 2A synchronous
2MHz boost LED driver featuring the
a single string of LEDs at 330mA up to 34V when V
between 7V and 28V. It runs down to 4V
I and can withstand V
LED
voltage lockout (OVLO) set for 28V for this application.
DC2247A runs at 2MHz switching frequency. Spread
spectrum frequency modulation (SSFM) can be turned
on with a simple jumper, reducing EMI. DC2247A comes
with low EMI features including optimized layout, SSFM,
and input and output filters. It passes CISPR 25 class 5
conducted and radiated EMI. It is protected against both
open and short LED conditions and reports the faults.
The LT3922 has an input voltage range from 2.8V to 36V.
Its internal synchronous 2A 40V switches allow up to 34V
of LEDs on the output with room for overvoltage protection
and overshoot during an open LED event. It has adjustable
switching frequency between 200kHz and 2.5MHz. It can
be synchronized to an external source or programmed with
SSFM for low EMI. The PWMTG high side PWM MOSFET
driver assists with short-circuit protection and versatility.
LT3922 can be configured as boost, boost-buck, or buck
mode LED driver and maintain all of its low-EMI, PWM
dimming and fault diagnostic features.
The LT3922 can be PWM dimmed with an external PWM
signal or an internally-generated PWM signal. DC2247A
has a jumper that can be set to switch between internally-
generated PWM signal, externally-generated PWM signal,
and no PWM signal (100% on). It can be analog dimmed
with a control voltage on its CTRL pin.
When run with both PWM dimming and SSFM, the spread
spectrum aligns itself with the PWM signal for flicker-free
operation.
LT 3922. It drives
®
with reduced
IN
as high as 36V, but has over-
IN
DEMO MANUAL DC2247A
36V 2A Synchronous
2MHz Boost LED Driver
Small ceramic input and output capacitors are used to
save space and cost. The board is designed with tiny, high
is frequency capacitors on both sides of the V
IN
a reduction in radiated EMI. The open LED overvoltage
protection uses the IC's constant voltage regulation loop
to regulate the output to approximately 37.5V if the LED
string is opened although it may reach almost 40V peak
during transient from running LEDs to open. There is a
protection diode from LED
ringing during a short circuit. The output current can be
monitored through the ISMON output pin.
Undervoltage and overvoltage lockout can be adjusted on
the circuit with a few simple resistor choices.
There is an EMI filter on the input of DC2247A. There is also
a small ferrite bead output filter. These filters, combined
with proper board layout and SSFM are effective to help
the PCB pass CISPR 25 class 5 conducted and radiated
EMI. Please follow the recommended layout and four-layer
thickness of DC2247A for low EMI applications.
The LT3922 data sheet gives a complete description of
the part, operation and applications information. The
data sheet must be read in conjunction with this Demo
Manual for DC2247A. The LT3922EUFD is assembled in a
28-lead plastic 4mm x 5mm QFN package with a thermally
enhanced ground pad. Proper board layout is essential
for maximum thermal performance. See the data sheet
section Layout Considerations.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2247A
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
LT3922
pins for
OUT
+
to GND to prevent negative
dc2247af
1